Process for the preparation of an uhmwpe homopolymer

ABSTRACT

A process for the preparation of an ultra-high molecular weight ethylene homopolymer having a MFR 21  of 0.01 g/10 min or less, said process comprising:
     (I) prepolymerising at least ethylene at a temperature of 0 to 90° C. in the presence of a heterogeneous Ziegler Natta catalyst to prepare an ethylene prepolymer having an Mw of 40,000 to 600,000 g/mol; and thereafter in the presence of the prepolymer and said catalyst;   (II) polymerising ethylene at a temperature of 55° C. or less, such as 20 to 55° C., to prepare said UHMW ethylene homopolymer; wherein the UHMW ethylene homopolymer comprises up to 8 wt.% of said prepolymer.

This invention concerns ultra high molecular weight (UHMW) polyethylenehomopolymers. In particular, the invention relates to a two-step processfor the manufacture of a UHMW polyethylene homopolymer that can beprocessed into elongated objects in the solid-state. The processrequires a pre-polymerisation step followed by a low temperature mainpolymerisation step. The invention also covers articles, preferablytapes, comprising the UHMW polyethylene homopolymer.

BACKGROUND

Ultra-high molecular weight polyethylene (UHMWPE) has a very highmolecular weight (typically Mw > 1 million) and therefore hasoutstanding physical and chemical properties, such as high abrasionresistance, high impact toughness, excellent resistance to corrosion andchemical degradation, resistance to cyclic fatigue and radiation.

However, increased molecular weight adversely affects the polymerprocessability due to increased number of entanglements per chain. Highentanglement density imparts excellent mechanical properties butrestricts the mobility of the polymer chains in the melt duringprocessing of the polymer into products. The high melt viscosity causespoor homogeneity of the processed product obtained from such a highmolecular weight material. Therefore, plasticization of the entanglementnetwork in an UHMWPE will allow easier processing with enhanced flowcharacteristics.

Lowering of physical contact (entanglements) between the neighbouringchains is therefore desired to enable processing of the UHMWPEhomopolymer, e.g. in a solid-state, into high tenacity/high modulusfibres and tapes.

A way to reduce contact between the chains (entanglements) of UHMWPE(having weight average molar mass greater than a million g/mol), is theuse of toxic solvents during the production of high modulus and highstrength fibers. In this process entanglement density (number ofcontacts per chain) is reduced by dissolving approximately 5 wt.% of thepolymer in 95 wt.% of a toxic and expensive solvent, such as decalin orxylene. This is not an attractive method for improving processability asthe solvent used has to be recovered and treated according to health andenvironmental requirements. Moreover, solvent residues are found in theend product. It is a feature of this invention that no toxic solventssuch as decalin or xylene are used and hence no residues of suchsolvents are present.

Another option to improve processability is the combination of theUHMWPE with a second material such as another polymer or a polymerprocessing aid. Zuo, Polym.Bull. (Heidelberg, Ger.) 58(4), 711-722 usedMMWPE as a compatibilizer/lubricant for UHMWPE/HDPE in two-step blendingprocess and reported improved processability, homogeneity and mechanicalperformance compared to direct processing. However, due to the high meltviscosity of UHMWPE, homogeneous mixing is demanding.

Aiello, R., Macromol.Mater.Eng., 286(3), 176-178, 2001 report reduceddie pressure and torque when up to 2% of processing aid (either liquidcrystalline polymer or fluoroelastomer) was added for extrusion ofUHMWPE. However, highly oriented chains required for high modulus andhigh strength of elongated objects could not be obtained.

Xie, European Polymer Journal, 43(8), 3480-3487, 2007 found asignificant reduction in melt viscosity of UHMWPE by blending withpolypropylene (PP) and poly(ethylene glycol) (PEG). They discovered theentanglement level of UHMWPE decreased with the addition of PP and/or asmall amount of PEG. However, due to molecular immiscibility orientedstructures required for high tensile modulus and high tensile strengthcould not be obtained.

In summary, in all of these processes UHMWPE chains are inherentlyentangled and the addition of the secondary component does not fullyresolve the processing limitations.

Therefore, attempts have been made to prepare UHMWPE with reduced numberof entanglements per chain. In general, to produce low-entangled UHMWPE,a single site homogeneous catalyst system with low catalystconcentration is preferred. For example, homogeneous single sitecatalyst systems having the active sites spaced apart in the reactionmedia. This spacing reduces the number of chain entanglements.

EP 1057835, describes a process for the polymerisation of ethylene at atemperature between -50 and 50° C. with a lanthanide catalyst. Thecatalyst is used for the production of polyethylene with a low degree ofchain entanglement due to crystallisation of individual chains duringpolymerisation.

US 2006/0142521 describes a process for the preparation of a shaped partof an ultrahigh molecular weight polyethylene (UHMWPE) wherein anultrahigh molecular weight polyethylene with a low degree ofentanglement is produced by polymerisation of ethylene at a temperatureof 225 to 325 K using an unsupported single-site catalyst.

WO 2010/139720 claims a process for manufacturing an ultra-highmolecular weight polyethylene wherein olefin monomers are contacted witha single site catalyst on a particulate carrier with a certain sitedensity. The concept is to keep the catalyst particles as far away fromeach other as possible to avoid entangling molecular chains and thisdemands a low site density, possibly at the expensive of polymer yield.

In WO 2015/121162, production of low-entangled high or ultrahighmolecular weight polyethylene in an unimodal slurry phase process isdescribed using Ziegler-Natta catalysts.

In Rastogi et al, Macromolecular Materials and Engineering 2003, 288,No. 12 p 964-70, UHMWPE is prepared with low disentanglement using a lowtemperature single site based polymerisation.

The process of the present invention improves upon the principlesoutlined in WO 2015/121162 by using a two-step process for thepreparation of the UHMWPE.

There remains a need to devise new processes for the manufacture ofUHMWPE polymers that can be processed into elongated objects in thesolid state. The present inventors have now found that it is possible toproduce UHMWPE homopolymers having a well-mixed low molar mass componenttogether with a lower degree of entanglement density using aheterogeneous Ziegler Natta catalyst in a two-step polymerisationprocess. The inventors have appreciated that a pre-polymerisation stepis critical to preparing a homogeneously mixed low molar mass componentin the final UHMWPE. Mixing is done in situ during the polymerisationprocess, and not by blending separate materials after polymerisation.Without being limited by theory, it is envisaged that the use of apre-polymerisation step helps to increase the distance between theactive sites, thus reducing the probability of chain entanglement duringthe polymerisation. Inventors also use a low temperature polymerisationstep. This approach allows increasing distance between the active sites,and when combined with the low temperature polymerisation process, thenucleation barrier required for crystallisation is suppressed. Thismeans that the process can have a crystallisation rate higher than thepolymerisation rate.

The pre-polymerisation step also favours the formation of lowentanglements in the non-crystalline domains of the semi-crystallinepolymer. In this regard, the pre-polymerisation temperature, and Mw ofthe polymer obtained in the pre-polymerisation step play an importantrole in reducing entanglement density in combination with homogeneousmixing of low molar mass component.

For an example, radius of gyration (R_(g)) increases with the increasingnumber of methylene units (N) covalently connected by bond length(l_(b)) i.e. R_(g) ~ (Nl_(b) ²)^(0.5), therefore influencing theseparation between the active sites in the pre-polymerisation step.

The pre-polymerisation is followed by a low temperature mainpolymerisation process. The use of a low temperature and a catalyst thatis subject to pre-polymerisation ensures that the polymer crystallisesand helps in the reduction of entanglements formation. A fundamentalpre-requisite for achieving the low-entangled state is that thecrystallisation rate must be higher than the polymerisation rate. Thepresent inventors appreciate that this can be achieved following theprocess described herein using a pre-polymerisation and subsequent lowtemperature polymerisation. The lower polymerisation temperaturesuppresses the nucleation barrier for enhancing the crystallisationrate.

SUMMARY OF INVENTION

Thus, viewed from a first aspect the invention provides a process forthe preparation of an ultra-high molecular weight ethylene homopolymerhaving a weight average molecular weight (Mw) of at least 1,000 kg/mol,said process comprising:

-   (I) pre-polymerising at least ethylene at a temperature of 0 to    90° C. in the presence of a heterogeneous Ziegler Natta catalyst to    prepare an ethylene prepolymer having a Mw of 40,000 to 600,000    g/mol; and thereafter in the presence of the prepolymer and said    catalyst;-   (II) polymerising ethylene at a temperature of 55° C. or less, such    as 20 to 55° C., to prepare said UHMW ethylene homopolymer; wherein    the UHMW ethylene homopolymer comprises up to 8 wt.% of said    prepolymer.

Alternatively viewed, the invention a process for the preparation of anultra-high molecular weight ethylene homopolymer having an MFR₂₁ of 0.01g/10 min or less, e.g. an unmeasurable MFR₂₁, said process comprising:

-   (I) pre-polymerising at least ethylene at a temperature of 0 to    90° C. in the presence of a heterogeneous Ziegler Natta catalyst to    prepare an ethylene prepolymer having a Mw of 40,000 to 600,000    g/mol; and thereafter in the presence of the prepolymer and said    catalyst;-   (II) polymerising ethylene at a temperature of 55° C. or less, such    as 20 to 55° C., to prepare said UHMW ethylene homopolymer; wherein    the UHMW ethylene homopolymer comprises up to 8 wt.% of said    prepolymer.

Viewed from another aspect the invention provides a process for thepreparation of an ultra-high molecular weight ethylene homopolymer, saidprocess comprising:

-   (I) prepolymerising at least ethylene in the presence of hydrogen at    a temperature of 0 to 90° C. and in the presence of a heterogeneous    Ziegler Natta catalyst to prepare an ethylene prepolymer having an    Mw of 40,000 to 600,000 g/mol;-   (II) flashing to remove any hydrogen from step (I); and thereafter    in the presence of the prepolymer and said catalyst;-   (III) polymerising ethylene in the absence of hydrogen at a    temperature of 55° C. or less, such as 20 to 55° C., to prepare an    UHMW ethylene homopolymer; wherein the UHMW ethylene homopolymer    comprises up to 8 wt.% of said prepolymer.

Viewed from another aspect the invention provides an ultra-highmolecular weight ethylene homopolymer obtained by a process ashereinbefore defined, e.g. one having a weight average molecular weight(Mw) of at least 1000 kg/mol or an MFR₂₁ of 0.01 g/10 min or less, suchas an unmeasurable MFR₂₁.

Viewed from another aspect the invention provides an ultra-highmolecular weight ethylene homopolymer having a weight average molecularweight (Mw) of at least 1000 kg/mol or an MFR₂₁ 0.01 g/10 min or lesshaving:

-   a peak melting point is 137 to 142.0° C.;-   ΔH_(melt) is 170 to 240 J/g; and-   a crystallinity of 60 to 80%. Preferably T_(onset) is in the range    of 134.0 to 139.0° C.

It is an important aspect of the invention that the UHMWPE can beprocessed into elongated objects in the solid state. The UHMWPE polymercan be compressed in the solid state, below the onset of meltingtemperature (e.g. in the temperature range not exceeding 137° C.),preferably below 135° C. The UHMWPE polymer can then be simultaneouslyrolled and stretched ideally below the peak melting temperature. Thesimultaneously rolled and stretched material could be drawn further at atemperature between 145 to 155° C., preferably between 147 to 152° C.,by more than 100 times its initial length under tension. The tension isrequired to overcome contraction of the sample i.e. raising the meltingtemperature by applying constraint.

The term ultrahigh molecular weight implies a Mw of at least 1,000kg/mol or alternatively a MFR₂₁ of the UHMWPE polymer of the inventionis 0.01 g/10 min (ISO1133, 190° C., 21.6 kg load) or less. Preferably,it is not possible to measure MFR₂₁ due to the very high Mw.

Viewed from another aspect the invention provides an article, preferablya tape, comprising the ultra-high molecular weight ethylene homopolymeras hereinbefore described.

Viewed from another aspect the invention provides the use of anultra-high molecular weight ethylene homopolymer as hereinbefore definedin the manufacture of an article, especially a tape.

DEFINITIONS

The tests for any claimed parameter are given in the “analytical tests”section of the text which precedes the examples.

The term heterogeneous Ziegler Natta (ZN) catalyst implies that the ZNcatalyst is insoluble in the medium used for the pre-polymerisation andpolymerisation. A solution of the catalyst in the reaction medium is notformed in the claimed process.

Wherever the term “Mw” is used herein, the weight average molecularweight is meant. Wherever the term “Mn” is used herein, the numberaverage molecular weight is meant.

The UHMWPE homopolymer of the invention is the polymer that is formedfrom the combination of the prepolymer and the polymer formed in themain polymerisation step.

The term main polymerisation is used herein to describe thepolymerisation step that occurs after the pre-polymerisation. Theprocess of the invention preferably consists of two steps, apre-polymerisation step followed by the polymerisation step.

It is an important aspect of the invention that the UHMWPE can beprocessed into elongated objects in the solid state. By elongated in thesolid state is meant that the UHMWPE polymer can be compressed in thesolid state, below the onset of melting temperature and then besimultaneously rolled and stretched below the peak melting temperature.

DETAILED DESCRIPTION OF INVENTION

The present invention relates to a process for the preparation of anUHMWPE homopolymer in a two-step process. The process requires apre-polymerisation step to prepare a prepolymer. Thereafter, the mainpolymerisation step takes place in the presence of the prepolymer andthe catalyst used to prepare that prepolymer. As explained below, thetwo step process is carefully controlled to ensure that the UHMWPEhomopolymer that is produced has a low level of entanglement.

A low degree of entanglement is also favoured by reducing the partialpressure of monomer present in the main polymerisation reactor, i.e. areduction in the monomer concentration.

Catalyst

The process of the invention relies on the use of a heterogeneousZiegler Natta (ZN) catalyst. As homogenous polymerisation is notindustrially viable due to very poor control over polymer morphology,the process of the invention is achieved using a heterogeneousZiegler-Natta (ZN) catalyst system, i.e. one in which the catalyst isnot dissolved in the reaction medium. Typically, therefore the catalystis supported.

Advantageously, the active sites on such a support are spaced apart toreduce chain entanglement. The required distance between the activesites on a judiciously chosen support can be achieved by introducing apre-polymerisation step, in particular one with the preferred featuresdefined herein. The reduced entanglements allow the ZN polymer to beprocessable in the solid state.

The catalyst is one which is preferably carried on a support. Generally,the ZN catalyst comprises at least a catalyst component formed from atransition metal compound of Group 4 to 6 of the Periodic Table (IUPAC,Nomenclature of Inorganic Chemistry, 1989), a metal compound of Group 1to 3 of the Periodic Table (IUPAC), optionally a compound of group 13 ofthe Periodic Table (IUPAC) and optionally an internal organic compound,like an internal electron donor.

A ZN catalyst may also comprise further catalyst component(s), such as acocatalyst and optionally external additives, like an external electrondonor.

Suitable Ziegler-Natta catalysts used in the present inventionpreferably contain a magnesium compound, an aluminium compound and atitanium compound supported on a particulate support.

The particulate support can be an inorganic oxide support, such assilica, alumina, titania, silica-alumina, silica-titania or a MgCl₂based support. Preferably, the support is silica or a MgCl₂ basedsupport, especially silica.

It is preferred if the particle size of the support is within certainlimits. In particular, the support, typically silica, may have anaverage particle (D50) from 3 to 30 µm, e.g. 5 to 30 µm, preferably from5 to 20 µm, more preferably or from 6 to 15 µm. The particle size of thesupport is important as it enables the formation of a catalyst in whichthe active sites are separated. It is known that avoiding entangledmolecular chains can be improved by using a catalyst in which the activesites are spaced apart. We have found that the use of a catalystsupport, in particular a silica catalyst support, having the particlesize described above, results in an ideal active site distribution whenused in the process of the invention. The combination of thepre-polymerisation step and the catalyst particle size, reduces thedensity of entanglement network and homogeneously mixes the low molarmass component polymerised in the pre-polymerisation step with the mainpolymer.

The presence of low entanglement density network in the solid state isidentified by the possibility of elongation by more than 100 times afterrolling and stretching of the compressed powder.

It is preferred if the ZN catalyst comprises a Group 2 metal compoundpreferably a magnesium compound. The magnesium compound is preferablythe reaction product of a magnesium dialkyl and an alcohol. The alcoholis preferably a linear or branched aliphatic monoalcohol. Preferably,the alcohol has from 6 to 16 carbon atoms. Branched alcohols areespecially preferred, and 2-ethyl-1-hexanol is one example of thepreferred alcohols. The magnesium dialkyl may be any compound ofmagnesium bonding to two alkyl groups, which may be the same ordifferent, such as C1-10 alkyl groups. Butyl-octyl magnesium is oneexample of the preferred magnesium dialkyls.

It is preferred if the ZN catalyst comprises an Al compound. Thealuminium compound is preferably a chlorine containing aluminium alkyl.The alkyl group may contain 1 to 12, preferably 1 to 8, more preferably1 to 4 carbon atoms. Especially preferred compounds are aluminium alkyldichlorides and aluminium alkyl sesquichlorides, like methyl aluminiumdichloride, ethyl aluminium dichloride or butyl aluminium dichloride,especially ethyl aluminium dichloride.

The transition metal compound of Group 4 to 6 metal is preferably acompound of Group 4 or 5 metal compound, like a titanium or vanadiumcompound, more preferably a halogen containing titanium compound, mostpreferably chlorine containing titanium compound. Especially preferredtitanium compound is titanium tetrachloride.

The catalyst can be prepared by sequentially contacting the carrier withthe above mentioned compounds, as described in EP 688794 or WO 99/51646.Alternatively, it can be prepared by first preparing a solution from thecomponents and then contacting the solution with a carrier, as describedin WO 01/55230.

Another group of suitable Ziegler-Natta catalysts contain a titaniumcompound together with a magnesium halide compound acting as a support.Thus, the catalyst contains a titanium compound and optionally a Group13 compound, for example an aluminium compound on a magnesium dihalide,like magnesium dichloride. Such catalysts are disclosed, for instance,in WO 2005/118655, EP 810235, WO2014/096296 and WO2016/097193.

A typical internal organic compound, if used, is chosen from thefollowing classes: ethers, esters, amines, ketones, alcohols, anhydridesor nitriles or mixtures thereof. Preferably the internal organiccompound is selected from ethers and esters, most preferably fromethers. Preferred ethers are of 2 to 20 carbon-atoms and especiallymono, di or multicyclic saturated or unsaturated ethers comprising 3 to6 ring atoms. Typical cyclic ethers suitable in the present invention,if used, are tetrahydrofuran (THF), substituted THF, like 2-methyl THF,di-cyclic ethers, like 2,2-di(2-tetrahydrofuryl)propane,2,2-di-(2-furan)-propane, or isomers or mixtures thereof. Internalorganic compounds are also often called as internal electron donors.

All these components are typically supported on the catalyst support.

Cocatalyst

The Ziegler-Natta catalyst can be used together with an activator. Thismay be supplied separately to the polymerisation process. Activator mayalso be called cocatalyst. Suitable activators are Group 13 metalcompounds, typically Group 13 alkyl compounds and especially aluminiumalkyl compounds, where the alkyl group contains 1 to 16 C-atoms. Thesecompounds include trialkyl aluminium compounds, such astrimethylaluminium, triethylaluminium, tri-isobutylaluminium,trihexylaluminium and tri-n-octylaluminium, alkyl aluminium halides,such as ethylaluminium dichloride, diethylaluminium chloride,ethylaluminium sesquichloride, dimethylaluminium chloride and the like.Especially preferred activators are trialkylaluminiums, of whichtriethylaluminium, trimethylaluminium and tri-isobutylaluminium areparticularly used.

The amount of the activator used depends on the specific catalyst andactivator. Typically triethylaluminium is used in such amount that themolar ratio of aluminium to the transition metal, like Al/Ti, is from 1to 1000, preferably from 3 to 100 and in particular from about 5 toabout 30 mol/mol.

External Electron Donors

The catalyst may comprise an external additive, like an externalelectron donor as a further catalyst component. This may be suppliedseparately to the polymerisation process. External electron donors aretypically used in propylene polymerisation, but known to be used inethylene polymerisation as well. Suitable external electron donors knownin the art include ethers, ketones, amines, alcohols, phenols,phosphines and silanes. Examples of these compounds are given, amongothers, in WO 95/32994, US 4107414, US 4186107, US 4226963, US 4347160,US 4382019, US 4435550, US 4465782, US 4472524, US 4473660, US 4522930,US 4530912, US 4532313, US 4560671 and US 4657882.

External electron donors consisting of organosilane compounds,containing Si—OCOR, Si—OR, and/or Si—NR₂ bonds, having silicon as thecentral atom, and R is an alkyl, alkenyl, aryl, arylalkyl or cycloalkylwith 1-20 carbon atoms are known in the art. Such compounds aredescribed in US 4472524, US 4522930, US 4560671, US 4581342, US 4657882,EP 45976, EP 45977 and EP1538167.

It is especially preferred to use silanes selected from compounds of thegeneral formula

wherein R^(a), R^(b) and R^(c) denote a hydrocarbyl radical, inparticular an alkyl or cycloalkyl group, and wherein p and q are numbersranging from 0 to 3 with the sum p + q being equal to or less than 3.R^(a), R^(b) and R^(c) can be chosen independently from one another andare linear, branched or cyclic hydrocarbyl groups having 1 to 12 carbonatoms, preferably R^(a), R^(b) and R^(c) are independently selected fromthe group consisting of methyl, ethyl, n-propyl, n-butyl, octyl,decanyl, iso-propyl, iso-butyl, iso-pentyl, tert.-butyl, tert.-amyl,neopentyl, cyclopentyl, cyclohexyl, methylcyclopentyl and cycloheptyl.Suitable examples of this type of silanes are e.g.(tert-butyl)₂Si(OCH₃)₂, (cyclohexyl)(methyl)Si(OCH₃)₂,(phenyl)₂Si(OCH₃)₂, (cyclopentyl)₂Si(OCH₃)₂, (CH₃)₂Si(OCH₃)₂,(CH₃)₂Si(OCH₂CH3)₂, (CH₃CH₂)₂Si(OCH₂CH3)₂ and (CH₃CH₂)₂Si(OCH3)₂.

As external donors may also be used silanes selected from compounds ofthe general formula

wherein R³ and R⁴ can be the same or different a represent a linear,branched or cyclic hydrocarbon group having 1 to 12 carbon atoms,preferably R³ and R⁴ are independently selected from the groupconsisting of methyl, ethyl, n-propyl, n-butyl, octyl, decanyl,iso-propyl, iso-butyl, iso-pentyl, tert.-butyl, tert.-amyl, neopentyl,cyclopentyl, cyclohexyl, methylcyclopentyl and cycloheptyl.

In one embodiment of the invention, the cocatalyst is fed to thepre-polymerisation reaction but no external electron donor is used. Theexternal electron donor is however fed to the main polymerisation step.

Polymerisation Process

The UHMW ethylene homopolymer may be produced in a polymerisationprocess that requires a pre-polymerisation process followed by a mainpolymerisation process.

Whilst the process may be effected in one polymerisation zone (in whichboth steps are carried out consecutively), it is preferred if theprocess uses at least two polymerisation zones, e.g. two polymerisationvessels. The skilled person will be familiar with the steps required totransfer the product and catalyst from a pre-polymerisation vessel tothe main polymerisation vessel.

The first polymerisation zone and the second polymerisation zone shouldbe connected in series. It will be appreciated that the first step ofthe process results in the formation of a prepolymer and an activatedcatalyst. Both the prepolymer and the activated catalyst are transferredfrom the pre-polymerisation stage into the main polymerisation. In anindustrial process this transfer may be continuous and the addition ofnew reactants and catalyst to the pre-polymerisation step may occurcontinuously.

It is preferred to remove the volatile reactants of thepre-polymerisation stage before introducing the product of thepre-polymerisation stage into the subsequent main polymerisation stage.A conventional flash step is therefore preferred as is well known in theart. It is preferred if no hydrogen is present in the mainpolymerisation.

Catalyst Feed

Catalyst is fed to the pre-polymerisation step. Fresh catalyst will notgenerally be fed to the main polymerisation although certain catalystcomponents may be added to the main polymerisation. All necessarycatalyst components are introduced to the prepolymerisation step. Wherethe cocatalyst is fed separately it is possible that only a part of thecocatalyst is introduced into the prepolymerisation stage and theremaining part into subsequent main polymerisation stages. Also in suchcases it is necessary to introduce so much cocatalyst into theprepolymerisation stage that a sufficient polymerisation reaction isobtained therein.

It may also be that external electron donor, which is not essential forcatalyst function, is fed into either the pre-polymerisation step, thesubsequent main polymerisation or both stages.

The catalyst may be transferred into the prepolymerisation zone by anymeans known in the art. It is thus possible to suspend the catalyst in adiluent and maintain it as homogeneous slurry. It is preferred if thecatalyst is supplied as an oil having a viscosity from 20 to 1500 mPa·sas diluent, as disclosed in WO-A-2006/063771. It is also possible to mixthe catalyst with a viscous mixture of grease and oil and feed theresultant paste into the prepolymerisation zone. Further still, it ispossible to let the catalyst settle and introduce portions of thusobtained catalyst mud into the polymerisation zone in a mannerdisclosed, for instance, in EP-A-428054. The skilled person is able tomanipulate the catalyst feed.

Pre-polymerisation

The process of the invention requires a prepolymerisation step. Thepurpose of the prepolymerisation is to polymerise a small amount ofmonomer to prepare a prepolymer and active catalyst. Typically, thisprocess is effected at a low temperature and/or a low monomerconcentration.

The prepolymerisation step may be conducted in slurry phase or in thegas phase. Preferably prepolymerisation is conducted in the slurryphase. Thus, the prepolymerisation step may be conducted in a dedicatedslurry reaction vessel.

The prepolymerisation is preferably conducted in the presence of aninert diluent, typically a hydrocarbon diluent such as methane, ethane,propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc.,or their mixtures. Preferably the diluent is a low-boiling hydrocarbonhaving from 1 to 4 carbon atoms or a mixture of such hydrocarbons.

The temperature in the prepolymerisation step is typically from 0 to 90°C., preferably from 20 to 80° C. and more preferably from 25 to 75° C.

The pressure is not critical and is typically from 1 to 150 bar,preferably from 10 to 100 bar, especially 20 to 40 bar.

The residence time in the pre-polymerisation step is approximately 1 to60 such as 1 to 30 minutes depending e.g. on the temperature. Residencetime may be e.g. 1 to 40 minutes, such as 5 to 30 minutes. The amount ofmonomer is typically such that from about 0.1 to 1000 grams of monomerper one gram of solid catalyst component is polymerised in theprepolymerisation step. As the person skilled in the art knows, thecatalyst particles recovered from a continuous prepolymerisation reactordo not all contain the same amount of prepolymer. Instead, each particlehas its own characteristic amount which depends on the residence time ofthat particle in the prepolymerisation reactor. As some particles remainin the reactor for a relatively long time and some for a relativelyshort time, then also the amount of prepolymer on different particles isdifferent and some individual particles may contain an amount ofprepolymer which is outside the above limits. However, the averageamount of prepolymer on the catalyst typically is within the limitsspecified above.

The pressure of ethylene within the pre-polymerisation reactor may be2.5 to 5.0 bar. Alternatively viewed, the partial pressure of ethylenein the pre-polymerisation step may be 8 to 15 % of the overall pressurein the reactor.

The pre-polymerisation process preferably produces an ethylenehomopolymer. In theory, in addition to ethylene monomer it is possibleto use one or more C3-10 alpha-olefin comonomers in theprepolymerisation step if desired. Suitable comonomers are, for example,1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene and their mixtures.

The molecular weight of the prepolymer may be controlled by hydrogen asis known in the art. In a preferred embodiment, hydrogen is presentduring the pre-polymerisation step. Hydrogen may form 1 to 5 mol% of thegaseous feed to pre-polymerisation step, such as 1.5 to 3 mol%.Alternatively viewed, the partial pressure of hydrogen in thepre-polymerisation step may be 1.3 to 8 % of the overall pressure in thereactor.

The Mw of the polymer formed during the prepolymerisation step may rangefrom 40,000 to 600,000 g/mol, preferably 100,000 to 600,000 g/mol, evenmore preferably at least 200,000 g/mol, such as 200,000 to 550,000g/mol.

The amount of prepolymer is up to 8 wt.% of the UHMWPE homopolymer as awhole, such as 0.5 to 8.0 wt%, e.g. 1.0 to 8 wt%, such as 1.0 to 6.0wt.%, especially 1.5 to 5.0 wt.%.

The pre-polymerisation temperature, the amount of prepolymer and the Mwof the prepolymer are all important factors in reducing the entanglementof the resulting UHMWPE. If the Mw of the prepolymer is too low and isproduced in higher amount, it will affect the overall Mw of the finalpolymer. In a most unlikely situation, the final polymer may not beUHMWPE. On the other hand, if the Mw of the prepolymer is too high, itwould lead to uncontrolled fragmentation in the polymerisation process.

We observe that a higher Mw prepolymer tends to improve the mechanicalproperties of the final polymer but reduces the processability. Reducingthe content of prepolymer tends to improve the mechanical properties ofthe final polymer but reduces the processability. The process of theinvention can therefore be tailored depending on the target desired.

As is well known, antistatic additives may be used to prevent theparticles from adhering to each other or the walls of the reactor, asdisclosed in WO-A-96/19503 and WO-A-96/32420.

The prepolymerisation process also improves the performance of thecatalyst in main polymerisation step. A problem with a nonprepolymerised catalyst is that the activity of the catalyst might betoo high. A high activity catalyst leads to rapid polymerisation. Aspreviously noted, a fundamental pre-requisite for achieving thenon-entangled state is that the crystallisation rate must be higher thanthe polymerisation rate. A non prepolymerised catalyst has a tendency tolead to high polymerisation rates and hence entanglement.

So the prepolymerisation step allows the eventual polymers to besolid-state processed into tapes having high modulus and high breakingtenacity. The prepolymerisation fraction and Mw strongly influences theresulting mechanical properties of the tapes produced. Thepolymerisation temperature and ethylene pressure are also important forthe synthesis of low entangled UHMWPE.

After the pre-polymerisation step, the volatiles present are preferablyremoved. Thus the reaction mixture is flashed to remove monomers,hydrogen and any other volatiles present. It is preferred if the mainpolymerisation step is effected in the absence of hydrogen.

It is preferred if the product of the pre-polymerisation is transferredfrom the pre-polymerisation vessel to the main polymerisation vessel,often a slurry loop reactor. The transfer process can be effected usingknown techniques. It will be appreciated that both the prepolymer andthe activated heterogeneous ZN catalyst are transferred to the mainpolymerisation stage. Of course, if the main polymerisation is effectedin the same vessel then no transfer is required. The main polymerisationcan be started once the flash step has been effected.

In WO2016/050774, an “offline” catalyst activation step is disclosed inwhich a catalyst is contacted with ethylene before being recovered andisolated in solid form. The catalyst contains around 14 wt% prepolymer.

In the present invention, it is preferred if there is no recovery of thecatalyst between prepolymerisation and main polymerisation steps. In oneembodiment, it is preferred if the process of prepolymerisation and thenmain polymerisation is continuous. There should be no isolation stepafter prepolymerisation.

Moreover, the amount of prepolymer required in the present invention issignifcnatly higher than amounts suggested in ‘774.

Main Polymerisation

The polymer that is formed in the main polymerisation is an ethylenehomopolymer. There is no comonomer present in the main polymerisation.

The main polymerisation step may be conducted in the slurry phase. Inthe slurry phase, the polymer particles formed in the polymerisation,together with the catalyst fragmented and dispersed within theparticles, are suspended in the fluid hydrocarbon. The slurry isagitated to enable the transfer of reactants from the fluid into theparticles. This is preferably a loop reactor.

The polymerisation usually takes place in an inert diluent, typically ahydrocarbon diluent such as methane, ethane, propane, n-butane,isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures.Preferably the diluent is a low-boiling hydrocarbon having from 1 to 4carbon atoms or a mixture of such hydrocarbons. An especially preferreddiluent is propane, possibly containing minor amount of methane, ethaneand/or butane.

The temperature in the main polymerisation step is important. Accordingto the process of the present invention the polymerisation temperatureis 55° C. or below, e.g. preferably in the range of 20 to 55° C.,especially in the range of 25 to 50° C., most especially 30 to 45° C.Without wishing to be limited by theory, it is believed that the lowtemperature ensures that crystallisation occurs before entanglement. Lowtemperature polymerisation therefore reduces entanglement in thepolymer. In one embodiment, the temperature in the main polymerisationstep is lower than that of the pre-polymerisation step.

The pressure in the main polymerisation step conducted in the slurryphase polymerisation is preferably in the range of 10 to 30 bar, such as10 to 25 bar.

Another important feature of the process of the invention is the amountof ethylene in the main polymerisation stage. Having a relatively lowethylene content appears to reduce chain entanglement. Low ethylenepartial pressure appears to reduce polymerisation rate. It is thuspreferred if the ethylene partial pressure within the mainpolymerisation zone is preferably 3 to 8 bar, such as 4 to 7 bar.Alternatively viewed, the ethylene partial pressure within the reactoris 10 to 35% of the overall pressure in the reactor. Thus if the reactorpressure is 30 bar, then the ethylene pressure might be 3 to 10.5 bar.

We have established that the best results are obtained with apolymerisation temperature of 30° C. and an ethylene partial pressure of5 to 7 bar, especially 5.5 to 6.5 bar, like 6 bar.

The residence time within the main polymerisation is preferably 30 to 90mins.

The slurry polymerisation may be conducted in any known reactor used forslurry polymerisation. Such reactors include a continuous stirred tankreactor and a loop reactor. It is especially preferred to conduct thepolymerisation in loop reactor. In such reactors the slurry iscirculated with a high velocity along a closed pipe by using acirculation pump. Loop reactors are generally known in the art andexamples are given, for instance, in US-A-4582816, US-A-3405109,US-A-3324093, EP-A-479186 and US-A-5391654.

The slurry may be withdrawn from the reactor either continuously orintermittently. A preferred way of intermittent withdrawal is the use ofsettling legs where slurry is allowed to concentrate before withdrawinga batch of the concentrated slurry from the reactor. The use of settlinglegs is disclosed, among others, in US-A-3374211, US-A-3242150 andEP-A-1310295. Continuous withdrawal is disclosed, among others, inEP-A-891990, EP-A-1415999, EP-A-1591460 and WO-A-2007/025640. Thecontinuous withdrawal is advantageously combined with a suitableconcentration method, as disclosed in EP-A-1310295 and EP-A-1591460.

The main polymerisation step contributes at least 92 wt.% of the UHMPEhomopolymer, such as 99 to 94 wt.%, especially 99 to 95 wt.%.

UHMWPE Homopolymer

After the main polymerisation step, the UHMWPE can be recovered and canbe further processed in the powder form. I.e. the polymer to beprocessed does not need to be pelletized, i.e. introduced in anextrusion process. The UHMWPE of the invention is a homopolymer. In thisregard, the polymer is regarded as a homopolymer as long as the mainpolymerisation step is a homopolymerisation. If a small amount ofcomonomer is used in the pre-polymerisation step, we still regard theresulting polymer as a homopolymer as the comonomer content is deminimis. Ideally however, both prepolymer and main polymer componentsare homopolymers.

The UHMW polyethylene homopolymer of the invention preferably has Mw ofat least 1,000,000 g/mol, preferably at least 1,200,000 g/mol, e.g.1,000 kg/mol to 4,000 kg/mol.

The Mw/Mn of the UHMW PE homopolymer may be in the range of 2 to 9,preferably 2 to 5.

The UHMW homopolymer is preferably bimodal as the pre-polymerisationstep accounts for the homogeneous mixing of the low molar mass componentin the matrix of UHMWPE. This means that rheological estimation of molarmass requires bimodal parameters, as described in (Talebi, S.;Duchateau, R.; Rastogi, S.; Kaschta, J.; Peters, G.W.M.; Lemstra, P.J.;Macromolecules 2010 43 (6), 2780-2788. Molar mass and molecular weightdetermination of UHMWPE synthesized using a living homogenous catalyst).Molar mass of the samples can be determined using the rheological dataacquired by performing frequency sweep experiments at 160° C.

The density of the UHMWPE homopolymer is typically in the range of 945to 980 kg/m³, such as 955 to 975 kg/m³.

The value of MFR₂₁ is typically less than 0.01 g/10 min, which indicatesa very high molecular weight of the polymer. The MFR₂₁ of the UHMWhomopolymer is preferably too low to be measured.

The UHMWPE homopolymer of the invention is one that contains low levelsof entanglement. Polymer chains can become entangled duringpolymerisation. This invention targets polymers that are not entangled(or with very low degree of entanglements) as these polymers exhibitmuch higher processability whilst retaining excellent mechanicalproperties.

When a polymer has a very high Mw, the increased number of entanglementsper chain produces a polymer that is very difficult to process in themelted state. It has a very high melt viscosity. A way to overcome thischallenge is the reduction of entanglements in the non-crystallineregions of the semi-crystalline state, i.e. solid state. The number ofentanglements in the non-crystalline region has to be reduced to theextent that the polymer can be processed without melting.

The process of the present invention leads to UHMWPE homopolymers withlow levels of entanglement in the solid state. The lower degree ofentanglements in the solid state allows the polymer powder obtained fromthe reactor to be compressed below the onset of melting temperature, andthen simultaneously rolled and stretched ideally below the peak meltingtemperature. The material can then be drawn further under tension bymore than 100 times the initial length of the compressed film.

In a preferred embodiment therefore the UHMWPE of the invention can becompressed in powder form below the below the melting temperature, andpreferably below the T_(onset) value of the polymer, e.g. below 136.0°C., ideally below 135° C. The compressed material can be pre-stretchedby rolling, preferably below the melting point of the polymer, e.g.below 139.0° C., ideally below 138° C. The rolled polymer often forms afilm. The rolled material can then be stretched, e.g. at a temperatureof 145-155° C. The stretching process can achieve a draw ratio of over100, such as 100 to 250.

To achieve the goal of low entanglement density in the non-crystallineregion we prepare the UHMWPE homopolymer in a low temperature process,the polymer crystallises before the chains have the chance to entangle.This means that the resulting polymer has certain characteristics thatare remarkable for such a high Mw polymer. The UHMWPE homopolymertherefore forms a further aspect of the invention.

The UHMWPE homopolymer of the invention is also stretchable. Entangledcommercial polymers tend to undergo brittle failure when stretched. Asthe chains are so intertwined in the non-crystalline region, the polymerdoes not undergo stretching easily.

A relatively low entangled polymer of the invention however can bestretched more readily. The presence of homogeneously mixed low molarmass component obtained in the pre-polymerisation step furtherfacilitates processing of the compressed powder.

The ultra-high molecular weight ethylene homopolymer may have a peakmelting point is 137 to 142.0° C., preferably 138.0 to 142.0° C.

The ultra-high molecular weight ethylene homopolymer may have a onsetmelting point is 132 to 137.0° C.

The ultra-high molecular weight ethylene homopolymer may have ΔH_(melt)of 170 to 240 J/g.

The ultra-high molecular weight ethylene homopolymer may have acrystallinity of 60 to 80%, preferably 60 to 70%.

The UHMW homopolymer of the invention may have a tensile strength value(N/tex) at a draw ratio of 200 of 2.6 N/tex or more, such as 2.6 to 3.5N/tex.

The UHMW homopolymer of the invention may have a tensile strength value(N/tex) at a draw ratio of 100 of 1.8 N/tex or more, such as 1.8 to 2.8N/tex.

The UHMW homopolymer of the invention may have a tensile strength value(N/tex) at a draw ratio of 100 or more of 1.8 N/tex or more.

The UHMW homopolymer of the invention may have a tensile modulus value(N/tex) at a draw ratio of 200 of 180 N/tex or more, such as 180 to 250N/tex.

The UHMW homopolymer of the invention may have a tensile modulus value(N/tex) at a draw ratio of 100 of 150 N/tex or more, such as 150 to 220N/tex.

The UHMW homopolymer of the invention may have a tensile strength value(N/tex) at a draw ratio of 100 or more of 150 N/tex or more.

Blends

The UHMWPE homopolymer of the invention can be used on its own invarious applications highlighted below or may be combined with otherpolymer components. In particular the UHMWPE homopolymer might becombined with HDPE.

The UHMWPE can also be combined with standard polymer additives as iswell known in the art. Two or more different UHMWPE homopolymers of theinvention may also be combined to form a blend.

Applications

The UHMW homopolymer of the invention can be used to make all manner ofarticles. They are of primary interest in the formation of tapes. It isa particular feature of the invention that the polymers may allow theformation of exceptionally strong tapes for applications where suchstrength is important such as in ballistic applications and medicalimplants.

UHMWPE homopolymer of the invention can be processed by compressionmoulding, screw extrusion or ram extrusion. Prior to solid statedrawing, the synthesised polymers can be compression moulded below theirmelting temperature. Tapes of the invention have high tensile modulusand high breaking tenacity.

It will be appreciated that the preferred features of the polymers ofthe invention as described herein can all be combined with each other inany way.

The invention will now be described with reference to the following nonlimiting examples and figures.

FIG. 1 shows the tensile modulus (N/tex) of comparative example 2 andinventive examples 1 to 3 as a function of draw ratio.

FIG. 2 shows the tensile strength (N/tex) of comparative example 2 andinventive examples 1 to 3 as a function of draw ratio.

FIG. 3 shows the reproducibility of the polymerisation of Inventiveexample 3 with the tensile strength and tensile modulus data.

Analytical Tests Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the melt viscosity ofthe polymer. The MFR is determined at 190° C. for polyethylene. The loadunder which the melt flow rate is determined is usually indicated as asubscript, for instance MFR₂ is measured under 2.16 kg load, MFR₅ ismeasured under 5 kg load or MFR₂₁ is measured under 21.6 kg load.

Density

Density of the polymer was measured according to ISO 1183 / 1872-2B.

Thermal Properties

All experiments are performed using a TA Q2000 differential scanningcalorimeter. Polyethylene samples are weighted in low mass aluminiumTzero pans and lids on a XS3DU Mettler Toledo precision balance(sensitivity of ±0.001 mg). Between 1-5 mg of sample is used forperforming DSC experiments. To avoid any thermal oxidation theexperiments are conducted under a nitrogen atmosphere with a flow rateof 50 mL/min. A heating-cooling-heating temperature ramp from 50 to 180°C. is performed at a linear rate of 10° C./min. The heat of fusion andpeak melting temperature are determined by integrating the melting peakfrom 100 to 160° C. using the sigmoidal horizontal baseline integrationoption in the universal analysis 2000 software.

The crystallinity is determined against the theoretical value of heat offusion of 100% crystalline PE of 293 J/g.

Molecular Weight

For the pre-polymerised material molecular weight averages, molecularweight distribution (Mn, Mw, Mz MWD)

Molecular weight averages (Mz, Mw and Mn), Molecular weight distribution(MWD) and its broadness, described by polydispersity index, PDI= Mw/Mn(wherein Mn is the number average molecular weight and Mw is the weightaverage molecular weight) were determined by Gel PermeationChromatography (GPC) according to ISO 16014-1:2003, ISO 16014-2:2003,ISO 16014-4:2003 and ASTM D 6474-12 using the following formulas:

$\begin{matrix}{M_{n} = \frac{\sum_{i = 1}^{N}\Lambda_{\mspace{2mu} i}}{\sum_{i = 1}^{N}\left( \frac{A_{i}}{M_{i}} \right)}} & \text{­­­(1)}\end{matrix}$

$\begin{matrix}{M_{w} = \frac{\sum_{i = 1}^{N}{\left. 〚 \right)\left( {\left( A〛 \right._{ i}x\mspace{6mu} M_{i}} \right)}}{\sum_{i = 1}^{N}A_{i}}} & \text{­­­(2)}\end{matrix}$

$\begin{matrix}{M_{z} = \frac{\sum_{i = 1}^{N}{\left. 〚 \right)\left( {\left( A〛 \right._{ i}x\mspace{6mu} M_{i}^{2}} \right)}}{\sum_{i = 1}^{N}\left( \frac{A_{i}}{M_{i}} \right)}} & \text{­­­(3)}\end{matrix}$

For a constant elution volume interval ΔV_(i), where A_(i), and M_(i)are the chromatographic peak slice area and polyolefin molecular weight(MW), respectively associated with the elution volume, V_(i), where N isequal to the number of data points obtained from the chromatogrambetween the integration limits.

A high temperature GPC instrument, equipped with either infrared (IR)detector (IR4 or IR5 from PolymerChar (Valencia, Spain) or differentialrefractometer (RI) from Agilent Technologies, equipped with 3 ×Agilent-PLgel Olexis and 1× Agilent-PLgel Olexis Guard columns was used.As the solvent and mobile phase 1,2,4-trichlorobenzene (TCB) stabilizedwith 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) was used. Thechromatographic system was operated at 160° C. and at a constant flowrate of 1 mL/min. 200 µL of sample solution was injected per analysis.Data collection was performed using either Agilent Cirrus softwareversion 3.3 or PolymerChar GPC-IR control software.

The column set was calibrated using universal calibration (according toISO 16014-2:2003) with 19 narrow MWD polystyrene (PS) standards in therange of 0.5 kg/mol to 11 500 kg/mol. The PS standards were dissolved atroom temperature over several hours. The conversion of the polystyrenepeak molecular weight to polyolefin molecular weights is accomplished byusing the Mark Houwink equation and the following Mark Houwinkconstants:

K_(PS) = 19 x 10⁻³mL/g, α_(PS) = 0.655

K_(PE) = 39 x 10⁻³mL/g,  α_(PE) = 0.725

K_(PP) = 19 x 10⁻³mL/g, α_(PP) = 0.725

A third order polynomial fit was used to fit the calibration data. Allsamples were prepared in the concentration range of 0.5 -1 mg/ml anddissolved at 160° C. for 2.5 hours for PP or 3 hours for PE undercontinuous gentle shaking.

Uniaxial Solid-State Deformation

A general procedure for the preparation of tapes is as follows: 25 g ofpolymer powder is poured into a mold with a cavity of 620 mm in lengthand 30 mm in width and compression-molded at 130 bar for 10 min to forma sheet. The sheet is preheated for at least 1 min and rolled with aCollin calander (diameter rolls: 250 mm, slit distance 0.15 mm, inletspeed 0.5 m/min). The tape is immediately stretched on a roll (speed 2.5m/min). The rolled and stretched tape is further stretched in two stepson a 50 cm long oil heated hot plate. The tape comes in contact with thehot plate after 20 cm from the entrance of the hot plate. The draw ratiois obtained by dividing specific weight of the sheet prior todeformation by the specific weight of the tape after stretching.

Tensile Testing

Tensile properties are measured using an Instron 5566 tensile tester atroom temperature (25 _C). To avoid any slippage, the side action gripclamps with flat jaw faces are used. The nominal gauge length of thespecimen is 100 mm, and the test is performed at a constant rate ofextension (crosshead travel rate) 50 mm/min. The breaking tenacity (ortensile strength) and modulus (segment between 0.3 and 0.4 N/tex) aredetermined from the force against displacement between the jaws.

EXAMPLES Catalyst Preparation -ZN1 Complex Preparation:

87 kg of toluene was added into the reactor. Then 45.5 kg Bomag A(Butyloctyl magnesium) in heptane was also added in the reactor. 161 kg99.8 % 2-ethyl-1-hexanol was then introduced into the reactor at a flowrate of 24-40 kg/h. The molar ratio between BOMAG-A and2-ethyl-1-hexanol was 1:1.83.

Solid Catalyst Component Preparation:

330 kg silica (calcined silica, Sylopol ® 2100) and pentane (0.12 kg/kgcarrier) were charged into a catalyst preparation reactor. Then EADC(Ethylaluminium dichloride) (2.66 mol/kg silica) was added into thereactor at a temperature below 40° C. during two hours and mixing wascontinued for one hour. The temperature during mixing was 40 -50° C.Then Mg complex prepared as described above was added (2.56 mol Mg/kgsilica) at 50° C. during two hours and mixing was continued at 40 -50°C. for one hour. 0.84 kg pentane/kg silica was added into the reactorand the slurry was stirred for 4 hours at the temperature of 40 -50° C.. Finally, TiCl₄ (1.47 mol/kg silica) was added during at least 1 hourat 55° C. to the reactor. The slurry was stirred at 50 - 60° C. for fivehours. The catalyst was then dried by purging with nitrogen. Molarcomposition of the ready catalyst is: Al/Mg/Ti = 1.5/1.4/0.8 (mol/kgsilica). This catalyst is called ZN1 herein.

Ethylene Homo-Polymerisation Procedure

The polymerisation experiments were carried out in a 3 L bench scalereactor. About 100 mg of ZN1 catalyst was used in all thepolymerisations and triethylaluminium (TEA) was used as co-catalyst withan Al/Ti ratio of 15.

Prepolymerisation Step

To an empty 3.0 L reactor 1250 ml of propane was fed to thepolymerisation reactor as a polymerisation medium. After addition of thereaction medium, hydrogen was introduced (0.4-2.5 Bar) after whichtemperature was increased to 70° C. A batch of ethylene (3.7 bar) wasadded, then reactor pressure was allowed to be stable at 0.2 bar ofoverpressure and stirring speed was increased to 450 rpm. Then thecatalyst and the co-catalyst were added through automatic feeding usingnitrogen and 100 ml of propane. The total reactor pressure was 30.0-33.5bar depending upon the amount of hydrogen used, which was maintained bycontinuous ethylene feed. The prepolymerisation was stopped after 1-5 gof ethylene was consumed (prepoly degree = 1-6%).

The Mw of the pre-polymerisation fraction and the relative amount ofpre-polymerisation fraction is presented in Table 1. Higher hydrogenconcentrations lead to lower Mw in the prepolymer. Longer residence timeleads to a higher wt.% of the prepolymer in the overall UHMWPE.

Polymerisation Step

After the desired degree of pre-polymerisation was achieved, thevolatile components were flushed and the reactor was cooled to 20° C.1500 ml of propane was fed to the polymerisation reactor as apolymerisation medium. The reactor was warmed to the desired temperature(30-40° C.). Dimethoxydimethylsilane as an external electron donor wasadded to the polymerisation reactor (mole ratio: Si/Ti = 1.25). A batchof ethylene (4-6 bar) was added, then reactor pressure was allowed to bestable at 0.2 bar of overpressure and stirring speed was increased to450 rpm. The total reactor pressure was 16-21 bar depending upon thepolymerisation temperature and ethylene pressure, which was maintainedby continuous ethylene feed. Polymerisation time was 60 min after whichthe polymerisation was stopped by venting off the monomer together withthe reaction medium. Activity of the catalyst was measured on the basisof the amount of polymer produced (Activities are in the range of 0.5 -2.0 kgPO/g Cat/h). The detailed conditions for the series of experimentsare listed in Table 1.

T_(onset), Tm, Width, ΔH and crystallinity results are disclosed inTable 2. The MFR₂₁ value is too low to be measured. The polymers of theinventive examples have densities of 955-957 kg/m³.

Comparative example 1 corresponds to example IE3 of WO2015/121162, i.e.prepared without any pre-polymerisation step. This polymer could not beprocessed in solid-state and was thus not used in further testing.

In comparative example 2 the Mw of the pre-polymer is only 20 × 10³g/mol and the monomer pressure in the main polymerization is 4 bar, i.e.below the preferred minimum pressure of 5 bar. The polymer ofcomparative example 2 was still processable in solid-state, but thetensile modulus and tensile strength are lower than the inventivepolymers for a given draw ratio as can be seen in the FIGS. 1 and 2 .

TABLE 1 Example Prepol. Fraction (wt. %) Prepol. Mw (10³ g/mol) MainPolym. Temp. (°C) Monom. Press. (bar) Processing in solid-state Comp. 10.0 - 40 4 No Comp. 2 6.0 20 40 4 Yes Inv. 1 3.0 150 30 6 Yes Inv. 2 1.5500 30 6 Yes Inv. 3* 3.0 150 30 6 Yes * Without External Donor

TABLE 2 Experiment T_(onset) (°C) T_(m) (°C) Width (°C) ΔH _(melt) ¹ J/gCrystallinity % Comp 2 133.7 138.8 4.4 191 65 Inv 1 136.3 139.8 3.1 18864 Inv 2 137.1 140.5 3.0 186 63 Inv 3 136.1 139.6 3.2 181 62 ¹Considering the melting enthalpy of 100% crystalline to be 293 J/g

Uniaxial Solid-State Deformation

After compression, the samples were pre-stretched by rolling below themelting point of the polymer as obtained from the reactor. Thecalendaring (rolling) step combined by stretching resulted in a drawratio of 10. The rolling step is applied to achieve homogeneity in thestrip and to avoid extrinsic failure during uniaxial drawing. The secondstretching step on the rolled and the prestretched tapes was carried outat approx. 148-153° C. Results are given in Table 3.

TABLE 3 Experiment Compression °C Rolling/Pre-stretching °C Stretching°C Failure Comp 2 133 131/136 148 No Inv 1 135 136/143 151 No Inv 2 136136/143 152 No Inv 3 134 136/143 151 No

1-19. (canceled)
 20. An ultra-high molecular weight ethylene homopolymerhaving an MFR₂₁ of 0.01 g/10 min or less having: a peak melting point is137 to 142.0° C.; ΔH_(melt) is 170 to 240 J/g; and a crystallinity of 60to 80%.
 21. The ultra-high molecular weight ethylene homopolymer asclaimed in claim 20 wherein the density of the homopolymer is in therange of 945 to 980 kg/m³.
 22. An ultra-high molecular weight ethylenehomopolymer as claimed in claim 20 wherein the density of thehomopolymer is in the range of 955 to 975 kg/m³.
 23. The ultra-highmolecular weight (UHMW) ethylene homopolymer as claimed in claim 20comprising: (I) an ethylene prepolymer component having an Mw of 40,000to 600,000 g/mol; (II) an UHMW ethylene homopolymer component; whereinthe UHMW ethylene homopolymer comprises up to 8 wt.% of said prepolymercomponent.
 24. An article comprising the ultra-high molecular weightethylene homopolymer as claimed in claim
 20. 25. The article of claim24, wherein the article is tape.